1. Field of Invention
The present invention generally relates to photovoltaic cells. More particularly, the invention relates to a solar cell having an improved conductive front grid-shaped contact configuration, and a method of forming same.
2. Summary of the Prior Art
Photovoltaic cells essentially comprise a semiconductor substrate of one conductivity type having a shallow P-N junction formed adjacent the front surface thereof. The cells require electrical contacts (generally metallic in composition, and sometimes referred to as "electrodes") on both their front and rear surfaces in contact with the semiconductor substrate in order to be able to recover an electrical current from the cells when they are exposed to solar radiation. These contacts are commonly made of aluminum, silver or nickel. For example, a common arrangement with solar cells having a silicon substrate is to make the rear contact of aluminum and the front contact of silver.
The contact on the front surface of the cell is generally made in the form of a grid, comprising an array of narrow, elongate, parallel fingers that extend in one direction, and at least one elongate bus that intersects the fingers at a right angle. The width, number and arrangement of the fingers is such that the area of the front surface adapted for exposure to solar radiation is maximized. Further, in order to improve the conversion efficiency of the cells, it has been found beneficial to provide a thin anti-reflection coating of a material such as silicon nitride or an oxide of silicon or titanium on the front surface of the cells.
Solar cells formed utilizing a semiconductor substrate having a shallow P-N junction adjacent its front surface, with that surface being coated with an insulating coating such as silicon nitride, therefore, are well known in the art. Such substrates sometimes will be referred to hereinafter as "solar cell blanks". It will be understood by those skilled in the art that a typical solar cell blank might consist of an EFG-grown silicon substrate of p-type conductivity having a P-N junction located about 0.5 microns from its front surface, and having a silicon nitride coating about 800 Angstroms thick on its front surface. Equivalent solar cell blanks also are well known. For example, single crystal silicon substrates, cast polycrystalline substrates, epitaxial silicon on metallurgical grade silicon or fine grain polysilicon layers formed by chemical or physical vapor deposition, can all be used in the formation of a solar cell blank. Similarly, n-type as well as p-type material may be used, and shapes other than flat stock are permissible, e.g., a circular piece of material, or substrates having an arcuate or polygonal cross-section.
The rear surface contact is commonly formed by coating substantially the entire rear surface of a solar cell blank with an aluminum paste, and thereafter, heating the coated solar cell blank so as to alloy the aluminum with the silicon substrate. Normally, the aluminum coating covers the entire rear surface with the exception of a small area adjacent the periphery of the rear surface. The exterior surface of the aluminum tends to oxidize, however, thereby increasing the resistance of a soldered contact between the aluminum surface and a tab used to connect the solar cell electrically to adjacent solar cells or an external electrical circuit. Accordingly, it has been found useful additionally to leave apertures through the aluminum coating in the central portion of the rear surface area. A silver containing ink then is used to fill these apertures and slightly overlap the adjacent aluminum layer. These silver areas create locations for the attachment of the tabs to the rear contact of the solar cell which are more efficient than direct attachment to the aluminum.
The grid-shaped contact on the front surface has been formed in various ways. For example, in some cases the grid pattern is formed on the front surface by screen printing or some other technology, and thereafter fired to complete the alloying of the metal particles contained in the ink to the silicon substrate. The substrate with the grid pattern alloyed thereto is then coated with the anti-reflective coating. A more direct approach of first coating the semiconductor substrate with the anti-reflective coating, and thereafter applying the grid contact also has been utilized in the art. To accomplish the latter objective, portions of the anti-reflective coating have in some cases been etched away so as to expose portions of the front surface of the semiconductor substrate in the desired grid electrode pattern. Thereafter, the front contact is deposited or otherwise formed on the front surface in the region where the anti-reflective coating has been etched away.
Another approach to the formation of the front contact on a solar cell blank is the so-called "fired-through" method. That method consists of the following steps: (1) applying a coating of a metal/glass frit ink or paste to the front surface of a solar cell blank in a predetermined pattern corresponding to the configuration of the desired grid electrode, and (2) heating the coated solar cell blank at a temperature and for a time sufficient to cause the metal/glass composition to dissolve the anti-reflection coating and to form an ohmic contact with the underlying front surface of the semiconductor substrate. The "fired through" method of forming contacts is illustrated by PCT Patent Application Publication WO 89/12312, published 14 Dec. 1989, based on U.S. patent application, Ser. No. 205304, filed 10 Jun. 1988 by Jack Hanoka for an Improved Method of Fabricating Contacts for Solar Cells. The concept of firing metal contacts through an anti-reflection dielectric coating also is disclosed in U.S. Pat. No. 4,737,197, issued to Y. Nagahara et al. for "Solar Cell with Paste Contact" .
The so-called "fired through" technique presupposes the use of thick film technology wherein a suitable paste or viscous ink forms a relatively thick metal-containing film on the solar cell blank which when fired will dissolve the anti-reflection coating and bond to the underlying silicon or other semiconductor. In U.S. patent application No. 666,334, filed 7 Mar. 1991 by Jack I. Hanoka and Scott E. Danielson entitled "Method and Apparatus for Forming Contacts", there is disclosed an improved method for direct writing such a thick ink film on the front surface of a solar cell blank or other substrate. In that method, the discharge orifice of the pen is located far enough above the moving surface of the solar cell blank that the pen does not contact the ink deposited on the blank. This in turn allows more efficient solar cell manufacturing operations, and the formation of finger contacts having greater aspect ratios (i.e., the ratio of finger height to finger width) than was theretofor possible. The resulting increase of finger height, without a corresponding increase in finger width, is desirable because the finger is thereby capable of carrying greater current without an increase in so-called "light shadowing" (i.e., the preclusion of solar radiation from reaching the front surface of the solar cell blank).
As alluded to above, the contact pattern on the front surface of the solar cell blank generally comprises at least one bus and an array of narrow, elongate, parallel fingers intersecting the bus at a right angle. This configuration optimizes the amount of radiant solar energy that can reach the front surface of the solar cell blank, while at the same time providing an efficient means to collect and transmit the current generated in the semiconductor substrate. Both the fingers and the bus have heretofore been alloyed directly to the semiconductor substrate. This is true both in the process wherein the grid contact is formed on the semiconductor front surface and later coated with an anti-reflective coating, and also in the process wherein the grid contact is formed on the front surface of a solar cell blank to directly contact the underlying semiconductor either through openings etched in the coating or by being fired through the coating. The resulting front grid-shaped electrode configuration has provided economy to the process because the entire metallization pattern (bus bars and fingers) can be printed at the same time in the same processing steps utilizing the same materials (generally metal containing inks).
Improvements to the electrical characteristics of the front solar radiation receiving sides of solar cells are constantly being sought in the art. Several of the better known ways to accomplish such improvements are the provision of better light trapping capability to the cell, reduction of the shadowing of the front surface of the solar cell blank by the grid pattern, and the reduction of both the ideal and non-ideal components of the emitter saturation current density at the front of the cell. The ideal emitter saturation current density is made up of three components: recombination in the emitter layer, recombination at the semiconductor/anti-reflective coating interface, and recombination at the metal/semiconductor interface. Significant increases in solar cell efficiencies will require reductions in all of the sources of recombination. In addition, advances in the art which will simplify manufacture and/or reduce costs without adverse effect upon presently attainable solar cell efficiency and/or life expectancy also are desirable.